A heat pump is a device which is used to pump heat from a source of heat at a particular temperature or energy level to a heat sink at a higher temperature or energy level than the source.
In practical applications, the design of a heat pump installation involves the selection of a particular heat transfer area for heat transmission of the evaporator and condenser to match the power capability of the motor-compressor unit which will be required to pump heat between a particular range of temperature differential between the heat source and heat sink.
In most instances, the work done by the compressor on the refrigerant will be sufficient to cause the temperatures of the evaporator and condenser coils to be sufficiently displaced from one another such that the compressor work done on the refrigerant is just sufficient to maintain the desired design temperature of the condenser and evaporator coils.
Under certain operating conditions, because of peculiar heat transfer conditions which may lie outside the design limits of the heat transfer coils of the condenser and evaporator, operation of the heat pump may have to be discontinued or modified to permit operation of the motor compressor.
Under certain circumstances in the operation of a heat pump in a situation where the energy levels of the heat source and heat sink are very close together, the motor compressor in attempting to do its rated work on the refrigerant fluid may cause the output pressure at the head of the compressor to escalate beyond design pressures in the unusual operating circumstances.
In large heat pump installations, some relief must be provided in order to prevent damage to the components of the heat pump installation until the operation of the heat pump is restored to normal. This relief may be in the form of some kind of unloader valve which opens under conditions where high head pressures occur in compressor operation, the opening of the unloader valve permits refrigerant to flow through the compressor without any work having been done on the refrigerant passing through the unloader valve and its bypass channel. It is necessary in high powered heat pump installations that some protective method must be found to provide relief for the high head pressures which periodically occur in abnormal operating conditions in order to prevent unscheduled shut down of the equipment or serious damage to the heat pump installation in the event some form of relief is not provided.